Group A rotaviruses are an important cause of diarrhea of human infants and the young of many other species. Two rotavirus outer capsid proteins, VP4 and VP7, influence virulence and are independent neutralization antigens. Fourteen VP7 serotypes (G types) have been identified of which four (G3, G5, G13 and G14) have been detected in equine rotaviruses, with G3 as the predominant serotype. Eleven VP4 serotypes (P types) are recognized among group A rotaviruses but the VP4 serotypes of equine rotaviruses have not been characterized. To develop VP4 serotyping reagents, panels of reassortants were generated with human rotavirus strain DS-1[G2] and each of five equine rotavirus strains (H2[G3], FI-14[G3], H1[G5], L338[G13], and FI-23[G14]). Reassortants containing the genomic segments encoding the equine rotavirus VP4 with at least VP7 from the human rotavirus strain were used to prepare hyperimmune sera. VP4 relationships among the equine viruses were established by plaque reduction neutralization assays using the hyperimmune sera. To examine the relationships between equine rotavirus VP4s and those of other species, the hyperimmune sera were tested for neutralizing activity with representative rotaviruses of all 11 currently defined VP4 serotypes. Cold-adaptation of rotaviruses may be associated with attenuation of virulence. In order to investigate the genetic basis for cold adaptation, propagation of equine rotavirus H2 in MA104 cells at progressively lower temperatures was achieved starting with routine culture at 37 degrees C and progressing stepwise downward in temperature to 30 degrees C, 28 degrees C, and 26 degrees C. Virus was triply plaque purified after the tenth passage at each temperature. Currently, the H2 virus is being adapted to grow at 25 degrees C. Plaque purified viruses have been examined for growth and plaquing ability in MA104 cells at temperatures ranging from 26 degrees C to 43 degrees C.